DESCRIPTION (Applicant's Abstract): Complex interactions of multiple genes expressed in the endoderm and mesoderm are necessary for differentiation and development of the gastrointestinal tract. The specialized and distinct epithelia of the esophagus, stomach, intestine and colon arise from the endoderm during embryogenesis. The unique characteristics of these organs are usually maintained throughout life, however, transdifferentiation or intestinal metaplasia can occur in which intestinal-type mucosa arises in the esophagus or stomach. The origin of this important human premalignant lesion is unknown but of clear clinical relevance. CDX1 and CDX2, intestine specific homeobox transcription factors, are candidate genes for directing development, differentiation and the maintenance of the intestinal phenotype. Moreover, CDX1 and CDX2, although not expressed in the normal stomach or esophagus, are expressed in intestinal metaplasia of the gastric and esophageal mucosa. The investigator hypothesizes that CDX1 and CDX2 direct the transdifferentiation program that occurs in intestinal metaplasia. This hypothesis will be addressed by interrelated specific aims, which employ transgenic mouse models to explore the effects of Cdxl and Cdx2 in the developing stomach and adult gastric and esophageal epithelia through misexpression of both genes. Specific aim 1 examines the biological consequences of Cdx expression in early embryogenesis. The coding regions of Cdxl and Cdx2 will be placed under the control of Hnf3gamma cis-regulatory elements, which will direct ectopic expression of Cdxl or Cdx2 in the endoderm of the developing stomach of transgenic mice. The applicant has already successfully expressed Cdx2 by this method and has evidence of intestinal type cells. Specific aim 2 utilizes a second transgenic mouse model to define the effect of Cdx expression on the differentiated esophageal and gastric mucosa by placing the coding regions of Cdxl or Cdx2 under the control of a tetracycline inducible promoter. The phenotype of both transgenic models will be examined for changes in morphology and gene expression. The expression of intestine specific genes will be determined and subsequent analysis will include subtractive hybridization to study in detail genes that are differentially expressed in tissue expressing the transgene. In addition, as intestinal metaplasia is a premalignant condition in humans, the animals will be assessed for increased susceptibility to cancer. In summary, the overall goal of this project is to elucidate the biological roles of Cdxl and Cdx2 in intestinal metaplasia and the molecular mechanisms of the transdifferentiation program induced by Cdxl and Cdx2.